At the outset of his "Principia," Sir Isaac Newton took the greatest care to impress upon his school that he did not use the word "attraction" with regard to the mutual action of bodies in a physical sense. To him it was, he said, a purely mathematical conception involving no consideration of real and primary physical causes. In one of the passages of his "Principia" (Defin. 8, B. I. Prop. 69,"Scholium"),he tells us plainly that, physically considered, attractions are rather impulses. In section XI. (Introduction) he expresses the opinion that "there is some subtle spirit by the force and action of which all movements of matter are determined" (see Mod. Mater., by Rev. W. F. Wilkinson); and in his third Letter to Bentley he says: "It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material,operate upon and affect other matter, without mutual contact, as it must do if gravitation, in the sense of Epicurus, be essential and inherent in it. . . . That gravity should be innate, inherent and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of anything else by and through which their action may be conveyed from one to another, is to me so great an absurdity that I believe no man, who has in philosophical matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my readers."